1. Field of the Invention
The present invention relates to a load sensor for determining the amount of a load on the basis of the degree of bending of a platelike strain-generating member having strain elements thereon, and more particularly, to a mounting structure for supporting a strain-generating member on a fixed shaft such as a bolt.
2. Description of the Related Art
FIG. 8 is an explanatory view of such a type of conventional load sensor. In the load sensor, strain elements 1 made of a thick film resistive material are mounted on a strain-generating member 2 made of a metal plate and having a bore 2a. A load point 2b, to which a load from an object to be measured is applied, is formed at the leading end of the strain-generating member 2. A bolt 3 is passed through the bore 2a, and is fixed, by using a nut 9 or the like, to a frame 4 or the like which serves as a mounting member. A pair of washers 5 and 6 shaped like a flat plate are fitted on the bolt 3. The rim of the bore 2a of the strain-generating member 2 is positioned so that it is disposed between the washers 5 and 6. A pair of nuts 7 and 8 are screwed on the bolt 3. The washer 6, the strain-generating member 2, and the washer 5 are fixedly clamped between the nuts 7 and 8 in a stacked state.
That is, in order to mount the strain-generating member 2 on the bolt 3, the nut 8 is screwed on the bolt 3, the washer 6, the strain-generating member 2, and the washer 5 are sequentially fitted on the bolt 3, the nut 7 is screwed on the bolt 3, and both of the nuts 7 and 8 are tightened. Since the washers 5 and 6, which sandwich the rim of the bore 2a of the strain-generating member 2, are thereby clamped between the nuts 7 and 8 in the axial direction of the bolt 3, the strain-generating member 2 can be firmly fixed to the bolt 3 via the washers 5 and 6.
In this type of load sensor, the strain-generating member 2 is shaped like a metal plate made of SUS or the like and has a thickness of, for example, approximately 2 mm. When a strong clamping force acts on the strain-generating member 2 in the thickness direction thereof, the strain-generating member 2, including the portion where the strain elements 1 are mounted, is slightly distorted, and this has an adverse effect on the properties of the strain elements 1. Therefore, in the conventional art in which the rim of the bore 2a of the strain-generating member 2 is tightly clamped by the washers 5 and 6, it is difficult to bring the surfaces of the strain-generating member 2 and the washers 5 and 6 into complete contact with each other. Since it is also difficult to bring the surfaces of the washers 5 and 6 and the nuts 7 and 8 into complete contact with each other, the portions of the strain-generating member 2 where the stress from the washers 5 and 6 are concentrated cannot be determined. For this reason, the degree of the distortion due to the clamping force substantially varies, and the properties of the strain elements 1 differ among products. That is, conventionally, even when equivalent loads are applied to the load point 2b, the output values of the strain elements 1 are apt to vary among products, and it is impossible to achieve high reliability.
The present invention has been made in view of the conventional circumstances, and it is an object of the present invention to provide a highly reliable load sensor in which nuts can be prevented from loosening with time, a strain-generating member can be reliably mounted, and the properties of strain elements do not vary among products.
In order to achieve the above object, according to an aspect of the present invention, there is provided a load sensor including a platelike strain-generating member having a bore formed at a distance from a load point on which a load acts, a strain element mounted on the strain-generating member between the load point and the bore, a fixed shaft for supporting the strain-generating member, and a spacer member fixed to the fixed shaft, wherein the spacer member has a shaft portion to be passed through the bore of the strain-generating member, and a flange portion extending from the shaft portion along the surface of the strain-generating member, and a strain-generating member holding space is formed between the flange portion and the rim of the bore so as to extend in the axial direction of the fixed shaft.
Since the strain-generating member having a thickness less than the height of the strain-generating member holding space is placed in the strain-generating member holding space, even when the spacer member is fixed to the fixed shaft by a strong pressing force, the pressing force does not directly act on the strain-generating member. For this reason, the strain-generating member is not subjected to an unreasonably large force when mounted, and variations in properties of the strain elements among products can be avoided. Moreover, even when the spacer member is mounted to the fixed shaft by a strong pressing force, this does not have any adverse effect on the detection accuracy, and therefore, prevention of loosening with time can be guaranteed. By placing the rim of the bore of the strain-generating member opposed to the spacer member so that there is a slight clearance therebetween, it is possible to prevent the displacement of the strain-generating member in the thickness direction thereof from adversely affecting detection data.
Preferably, the fixed shaft is a threaded bolt, and the spacer member is fixed by nuts fastened to the bolt. This makes is possible to easily and reliably perform the mounting operation. In this case, the spacer member may include a tube spacer having a length greater than the thickness of the strain-generating member so that it can be passed through the bore, and a pair of platelike washers for clamping the tube spacer by the nuts. Alternatively, the spacer member may include a pair of convex washers each having a cylindrical portion at the center thereof. The cylindrical portions of the washers may be clamped by the nuts, and the bore of the strain-generating member may be fitted on the cylindrical portions.
Preferably, the thickness of the portion of the strain-generating member adjacent to the bore is set to be greater than the thickness of the portion of the strain-generating portion where the strain element is mounted. In this case, when the strain-generating member is bent as a result of an application of a load to the load point, deformation starts on the border between the thin portion and the thick portion. Therefore, even when the holding position of the strain-generating member is displaced to some extent, the starting point of deformation does not change, and the reliability is further improved.
Preferably, the strain-generating member holding space is filled with a fixing adhesive. In this case, since the holding position of the strain-generating member is not displaced, the starting point of deformation is stabilized, and the reliability is further improved.
According to another aspect of the present invention, there is provided a load sensor including a platelike strain-generating member having a bore formed at a distance from a load point on which a load acts, a strain element mounted on the strain-generating member between the load point and the bore, a fixed shaft passed through the bore of the strain-generating member, a pair of spacer members fitted on the fixed shaft with the strain-generating member therebetween, and a fixing means for fixing the spacer members by pressing the spacer members in the axial direction of the fixed shaft, wherein a concave or convex pressed portion is formed at a predetermined position on the periphery of the bore of the strain-generating member, the pressed portion is clamped between the spacer members, and the front and back surfaces of the strain-generating member except the pressed portion are held spaced from the spacer members.
In this case, since a clamping force is applied only to a predetermined position on the periphery of the bore of the strain-generating member, even when the strain-generating member is slightly distorted due to clamping, the degree of distortion does not vary among products. Consequently, the properties of the strain elements do not vary among products, and reliability is improved.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.